As a good practice, post your code, formatted with the </> icon.
As a second good practice, test your yaml in the dev tools
If you test Reading now is: {{ states', it would have tell you what was wrong because states is a function that require a parameter.
And parameters are surrounded by parenthesis.
Why is your sensor not in the list? I can’t tell because there is a scroll bar and s is after d in the alphabet.
EDIT: As an exemple to help you, here are a few tests I did especially for you
{{ state_attr('climate.thermostat','temperature') }}
The time is {{ now().strftime('%-I') }} o'clock
Reading now is: {{ states('sensor.annif_olivier') }}
Ok, if you go to developer tools > states (My HA is not in English, you might have to search)
Then, in the filter entities box, start typing smart_ does it show? If yes, copy/paste